A high-pressure mercury lamp, a metal halide lamp, and other discharge lamps may be used as the light source of a projector. In such discharge lamps, discharging causes electrode consumption, which changes the shapes of the electrodes. A plurality of protrusions that may grow at the tips of the electrodes or irregular consumption that may proceed in the electrode bodies causes an arc start point shift and an arc length change. These phenomena may lead to a reduction in luminance and life of the discharge lamp.
To solve the above problems, U.S. Pat. No. 6,815,907 proposes a method in which a discharge lamp driving AC current undergoes pulse width modulation with the absolute value thereof maintained at a substantially fixed level.
However, when the pulse width modulation of the discharge lamp driving AC current is carried out at a high frequency, such as in U.S. Pat. No. 6,815,907, steady convection is disadvantageously formed in association with light emission in the discharge lamp, resulting in uneven consumption of the electrodes and uneven precipitation of the electrode material in some cases.
Further, when a discharge lamp is used in a projector or any other similar apparatus, a user's dimming operation and mode switching operation (for example, switching between a high-luminance mode driven at a high power level and a long-life mode driven at a low power level) may change the driving power. To prevent the formation of steady convection associated with light emission in the discharge lamp, it is necessary to change the electrode temperature within the largest possible range. However, since the electrode temperature is proportional to the power supplied to the discharge lamp, particular care should be taken to significantly change the electrode temperature when the discharge lamp is driven at a low power.